Chrominance-Based Keys

Chrominance-based keys use chrominance (or color) to define the transparent areas of a clip. Chrominance-based keys are commonly used to composite moving subjects that were shot against a colored background, usually a blue or green backdrop called a bluescreen or greenscreen, respectively. Provided the subject doesn't contain the key color, a chrominance-based key can remove the background while leaving the subject opaque. In fact, blue and green are standard key colors because they are relatively absent from human skin tones. Without the key color to differentiate the background from the subject, it would be difficult, if not impossible, to perfectly separate a moving subject from the background (Figure 13.32). Chrominance-based key types include Chroma, Blue Screen, Green Screen, RGB Difference, and Non-Red. The following sections explain in detail how to apply the chroma key and summarize the other color-based keys.